There is no doubt that the remarkable success of checkpoint blockade and of adoptive T cell transfer in the clinic has positioned immunotherapy firmly as the fourth pillar of cancer treatment, next to surgery, radiotherapy, and chemotherapy and created opportunities and challenges in equal measures. Some of the most pressing questions we are facing include 1) relatively limited response rates, 2) the emergence of immune-related side effects, and 3) a lack of predictive biomarkers. While combining immunotherapies, within and across modalities will address some of these challenges, new ones are likely to arise. Certainly, radiation therapy combined with immunotherapy can yield promising responses, but our understanding of the mechanisms involved is very limited. Perhaps the most compelling aspect of radiation therapy lies in its cytotoxic nature and its ability to create a hub of immunogenic tumor cell death which is characterized by a unique biomarker signature, most notably cell surface exposure of calreticulin. Dendritic cells have the exceptional ability to sense danger signals and relay them to the adaptive arm of the immune system by way of licensing cytotoxic CD8 T to seek out their target and kill. The important issue is to understand when this happens, when it doesn't happen and if it translates into systemic immunity and the ability to fight micrometastatic disease. However, our tools to address these questions are totally inadequate. Indeed, this proposal aims to develop powerful new tools that allow us to interrogate the tumor-host interface by noninvasively imaging immune cells during an active immune response using engineered antibody fragments and immunoPET. A novel calreticulin immunoPET probe will be developed, and evaluated alongside an existing CD8 immunoPET probe for the ability to assess the induction of immunogenic cell death and the influx and expansion of effector cells. The goal is to demonstrate that CD8 immunoPET can be used to monitor tumor immune responses during radiation therapy; that calreticulin immunoPET can provide an early non- invasive biomarker of immunogenic cell death and predict subsequent CD8 T cell infiltration in tumors, and that the combination of calreticulin and CD8 imaging can be used to optimize RT dose/fractionation schedules so as to create the optimal adjuvant to immunotherapy.